A conventional cardiac pacemaker is an implantable battery-powered electronic device that responds to sensed cardiac events and elapsed time intervals by changing its functional states so as to properly interpret sensed data and deliver pacing pulses to the heart at appropriate times. The pacing pulses are delivered through a lead made up of electrodes on a catheter or wire that connects the pacemaker to the heart. Some electronic devices, called implantable cardioverter-defibrillators, are capable of delivering electrical shocks, rather than small-intensity pacing stimuli, in order to cardiovert or defibrillate the heart. The term “pacemaker” as used herein, however, should be taken to mean both pacemakers and any device with a pacemaking function, such as an implantable cardioverter/defibrillator with a pacemaker incorporated therein.
Pacemakers can generally operate in a variety of modes, generally designated by a letter code of three positions where each letter in the code refers to a specific function of the pacemaker. The first letter refers to which heart chambers are paced and which may be an A (for atrium), a V (for ventricle), D (for both chambers), or O (for none). The second letter refers to which chambers are sensed by the pacemaker's sensing channels and uses the same letter designations as used for pacing. The third letter refers to the pacemaker's response to a sensed P wave from the atrium or an R wave from the ventricle and may be an I (for inhibited), T (for triggered), D (for dual in which both triggering and inhibition are used), and O (for no response). Modem pacemakers are typically programmable so that they can operate in any mode which the physical configuration of the device will allow. Additional sensing of physiological data allows some pacemakers to change the rate at which they pace the heart in accordance with some parameter correlated to metabolic demand. Such pacemakers, which are the primary subject of the present invention, are called rate-adaptive pacemakers and are designated by a fourth letter added to the three-letter code, R.
The most common condition for which pacemakers are used is the treatment of bradycardia. Permanent pacing for bradycardia is indicated in patients with symptomatic bradycardia of any type as long as it is likely to be permanent or recurrent and is not associated with a transient condition from which the patient may recover. Atrio-ventricular conduction defects (i.e., AV block) that are fixed or intermittent and sick sinus syndrome represent the most common indications for permanent pacing. In chronotropically competent patients in need of ventricular pacing, atrial triggered modes such as DDD or VDD are desirable because they allow the pacing to track the physiologically normal atrial rhythm, which causes cardiac output to be responsive to the metabolic needs of the body. Atrial triggering modes are contraindicated, however, in patients prone to atrial fibrillation or flutter or in whom a reliable atrial sense cannot be obtained. In the former case, the ventricles will be paced at too high a rate. Failing to sense an atrial P wave, on the other hand, results in a loss of atrial tracking which can lead to negative hemodynamic effects because the pacemaker then reverts to its minimum ventricular pacing rate. In pacemaker patients who are chronotropically incompetent (e.g., sinus node dysfunction) or in whom atrial-triggered modes such as DDD and VDD are contraindicated, the heart rate is dictated solely by the pacemaker in the absence of faster intrinsic cardiac activity. That pacing rate is determined by the programmed escape intervals of the pacemaker and is referred to as the lower rate limit or LRL.
Pacing the heart at a fixed rate as determined by the LRL setting of the pacemaker, however, does not allow the heart rate to increase with increased metabolic demand. Cardiac output is determined by two factors, the stroke volume and heart rate, with the latter being the primary determinant. Although stroke volume can be increased during exercise (e.g., due to increased venous return and increased myocardial contractility), the resulting increase in cardiac output is usually not sufficient to meet the body's metabolic needs unless the heart rate is also increased. If the heart is paced at a constant rate, as for example by a VVI pacemaker, severe limitations are imposed upon the patient with respect to lifestyle and activities. It is to overcome these limitations and improve the quality of life of such patients that rate-adaptive pacemakers have been developed.
The body's normal regulatory mechanisms act so as to increase cardiac output when the metabolic rate is increased due to an increased exertion level in order to transport more oxygen and remove more waste products. One way to control the rate of a pacemaker, therefore, is to measure the metabolic rate of the body and vary the pacing rate in accordance with the measurement. Metabolic rate can effectively be directly measured by, for example, sensing blood pH or blood oxygen saturation. Practical problems with implementing pacemakers controlled by such direct measurements, however, have led to the development of pacemakers that are rate-controlled in accordance with physiological variables that are indirectly reflective of the body's metabolic rate such as body temperature or respiratory rate. (See, e.g., U.S. Pat. No. 5,376,106 issued to Stahmann et al. and assigned to Cardiac Pacemakers, Inc., the disclosure of which is hereby incorporated by reference.) Measuring respiratory rate, for example, estimates oxygen consumption. A better approximation to oxygen consumption is the minute ventilation, however, which is the product of ventilation rate and tidal volume. An even more indirect measurement of metabolic rate is the measurement of body activity or motion with either an accelerometer or vibration sensor. The activity-sensing pacemaker uses an accelerometer or microphone-like sensor inside the pacemaker case that responds to motion or mechanical vibrations of the body by producing electrical signals proportional to the patient's level of physical activity. More complex rate-responsive systems incorporate multiple sensors that compensate for the deficits of specific individual sensors. All of the above-mentioned sensors, however, are for the purpose of ascertaining the exertion level of the patient and changing the heart rate in accordance therewith.
In such rate-adaptive pacemakers that vary the pacing rate in accordance with a measured exertion level, the control system is generally implemented as an open-loop controller that maps a particular exertion level to one particular heart rate, termed the sensor-indicated rate (SIR). Various parameters are set in order to fit the control system to the individual patient, including minimal and maximal heart rate and responsiveness. Minimal and maximal heart rate settings are primarily for patient safety, and the responsiveness of the pacemaker determines how much change in heart rate results from a given change in exertion level. An under-responsive pacemaker will unnecessarily limit exercise duration and intensity in the patient because the heart rate will not increase enough to match metabolic demand, while an over-responsive pacemaker will lead to palpitations and patient discomfort. Control parameters are generally set in conventional rate-adaptive pacemakers after implantation and during clinical visits according to a fixed formula or as a result of exercise testing. There is a need for rate-adaptive pacemakers that automatically adjust control parameters in accordance with a patient's changing physical condition so as to reduce the need for follow-up clinical visits and extensive testing.